Fix Web UI on Polymer 3.3.0

Since https://github.com/Polymer/polymer/releases/tag/v3.3.0 released on June 24th, the following tests are broken:

• create_doc.feature
• cloud.feature

It appears that web ui is broken everywhere we use * _attachDom* helper:

• https://github.com/nuxeo/nuxeo-web-ui/blob/master/elements/nuxeo-document-creation/nuxeo-document-creation-behavior.js#L85
• https://github.com/nuxeo/nuxeo-web-ui/blob/master/elements/nuxeo-cloud-services/nuxeo-tokens-behavior.js#L49

indeed they touched the _attachDom helper in latest release:

• https://github.com/Polymer/polymer/commit/3af9f3407b492156a78d850cb373219c8b4b03f8
• https://github.com/Polymer/polymer/commit/1aeaa8016f9baa34af845d2371cbf0b5856c5266
• https://github.com/Polymer/polymer/commit/3af9f3407b492156a78d850cb373219c8b4b03f8